An embodiment of the present invention is directed to a method of adjusting the emission rate of radiation from a source of radiation, such as an X-ray tube. An embodiment of the invention is directed to a method of adjusting the emission rate of radiation from a source of radiation that may be used in medical applications.
The operation of an X-ray tube is governed by the high voltage applied between an anode and a cathode of this tube, as well as by the electric heating current with which a filament of the cathode is taken to high temperature. According to the principle of X-ray emission, the electrons are extracted from the cathode and projected at high speed into the anode. The anode target, which is struck by these electrons, then emits X-rays, which can be used to produce X-ray exposures, or more generally X-ray images. The high voltage applied is directly related to the energy of the X-photons emitted.
Given the homogeneity of the target material of the anode, the variations in the power supply high voltage when the exposure or image is taken, as well as the statistical phenomenon of X-ray production, the X-rays are emitted with a broad spectrum. There are known ways of filtering them by means of filters interposed in the path of the radiation before it reaches the body to be irradiated.
The nature of the X-rays, and their energy, depends on the type of image to be taken. Certain interposed tissues to be imaged, especially the tissues of the human body, indeed have different X-ray absorption coefficients for different X-photon energy values. There are therefore known ways by which a practitioner, in an X-ray examination, will set the value of the high voltage.
Another parameter is the quality of an image to be produced and the rate of emission of the X-rays from the tube. Development on a detector is a cumulative energy phenomenon although it is not linear. The higher the emission rate, the greater the speed at which the mean dose to be injected will be obtained. In particular, for cardiac type examinations for which such speeds are necessary, it is desirable to control the quantity of photons emitted per time unit. In practice, there is a direct relationship between the quantity of X-photons emitted and the number of electrons that strike the anode. However, the number of these electrons depends, firstly, on the heating current. The greater the extent to which the cathode is excited by the heating current, the greater the number of free electrons likely to be liberated. Furthermore, the greater the high voltage between the anode and the cathode, the greater the statistical likelihood that this phenomenon of liberation will occur. Ultimately, the emission rate of X-rays from the tube depends on the heating current and the high voltage.
In the prior art, the method used to take account of these cross influences comprised calibrating the apparatus and determining the tube current, and therefore the rate of emission of the X-rays emitted for a set of high-voltage values, parameterized by a set of heating current values.
A drawback of this calibration method is that the operation of the tube is ensured only for the points of calibration. It is not truly possible, given the complexity of the phenomenon, to envisage an interpolation between the points of calibration. The possibility of such an interpolation is especially low as the specifications require that the emission rates requested by the practitioners should be provided with a relatively low tolerance of about 10%. Owing to the disparities of manufacture in a production line, and owing to the aging of the tubes, it is not long before the tolerance of 10% is barely met for the points of calibration. This tolerance is met to an even smaller degree for the points of interpolation.
Other methods used to deduce a real value of the tube current from values of heating current and high voltage applied to the x-ray tube are analytical methods based on theoretical models of the different phenomena involved in the production of the X-rays. These methods however provide no solutions, whether to the problems of disparities or to those of aging. Furthermore, they are complicated to implement that they are used only in experimental tubes and not in tubes of standard production.